1. Field of the Invention
The present invention relates to a multibeam radar system for use as a distance/speed detector in a collision alarm system on motor vehicles, and more particularly to a multibeam radar antenna and a multibeam radar module for radiating a plurality of radar beams in respective different directions with adjacent ones of the radar beams overlapping each other.
2. Description of the Prior Art
FM millimeter-wave radar devices for use as distance/speed detectors in collision alarm systems on motor vehicles are known from "Radar technology" published by the Society of Electronic Information Communications. The known FM millimeter-wave radar devices radiate a signal whose frequency increases or decreases in a triangular wave pattern with time forwardly of the motor vehicle and receive an echo signal reflected by another motor vehicle running ahead. The received echo signal is mixed with the transmitted signal, generating a beat signal. The distance up to the motor vehicle running ahead and the speed thereof are detected from the frequency of the beat signal.
For an FM radar system to be used as an automobile radar system, the FM radar system as installed on a motor vehicle running along a lane is required to detect a distance highly accurately within a range of 100 m and also to scan a road horizontally with high-frequency beams for monitoring motor vehicles running along left and right lanes in front of its own motor vehicle. This is because it is possible for any one of the motor vehicles running ahead along the left and right lanes to enter the lane of its own motor vehicle, and when one of the motor vehicles enters the lane, the speed of the motor vehicle equipped with the FM radar system has to be reduced to avoid collision.
Radar systems or apparatus for monitoring horizontal objects in front of a motor vehicle are disclosed in U.S. Pat. Nos. 5,008,678 and 5,486,832.
The radar system disclosed in U.S. Pat. No. 5,008,678 employs planar microstrip Butler matrixes for electronically scanning radar beams. Because of the planar microstrip Butler matrixes, however, the disclosed radar system is relatively expensive to manufacture. It is important that radar systems for use as radar sensors on motor vehicles be reduced in cost as they also find applications in radar automatic cruise control systems (ACC) and radar brake systems.
The radar apparatus revealed in U.S. Pat. No. 5,486,832 continuously radiates from a transmit antenna a broad fixed beam forwardly of the motor vehicle which carries the radar apparatus, and switches a plurality of receive antenna elements sequentially to receive a reflected beam. According the disclosure, the transmitted beam is not scanned, but the receive antenna elements are sequentially switched to receive the reflected beam for thereby monitoring an obstacle horizontally ahead of the motor vehicle. One drawback of the disclosed radar apparatus is that the transmit antenna and the receive antenna elements are located in separate positions. A dielectric lens used by the radar apparatus for converging a high-frequency beam is generally costly to manufacture. The cost of the radar apparatus will be high if independent lenses are used to transmit and receive a high-frequency beam.
Radar systems for motor vehicles are mostly mounted on front bumpers, and hence should be small in size. In addition, the radar systems for motor vehicles are required to have an increased azimuth resolution. If the width of the high-frequency beam is reduced to increase the azimuth resolution, then the dielectric lens has to have an increased diameter. According to the radar apparatus disclosed in U.S. Pat. No. 5,486,832, the entire antenna assembly is large because the transmit antenna and the receive antenna elements are fully independent of each other. The higher the azimuth resolution, the greater the antenna assembly, making it more difficult to install the radar apparatus on front bumpers.
It is preferable to reduce the width of the beam radiated from the transmit antenna for the purpose of minimizing interferences between radar apparatus on motor vehicles. However, the radar apparatus disclosed in U.S. Pat. No. 5,486,832 is susceptible to such interferences because a broad fixed beam is continuously radiated from the transmit antenna.